The immune correlates analysis of the RV144 vaccine trial, demonstrated 31% vaccine efficacy (60.5% vaccine efficacy through the first twelve months after vaccination). V1/V2 IgG antibodies correlated with decreased risk of HIV-1 infection, whereas high levels of envelope-specific IgA antibodies correlated with increased risk of infection. Our lab showed that IgA could block the protective effects of IgG through competition of the same binding sites for antibody dependent cellular cytotoxicity (ADCC). In contrast to a role for IgA in diminishing protection, a non-human primate study revealed that IgA1 protected from infection better than paratope-matched IgA2 or IgG upon intrarectal SHIV challenge. Taken together, these studies suggest a dual role of vaccine-induced IgA where IgA may diminish protection by competing with protective IgG or provide a potentially protective role by acting to prevent HIV-1 acquisition. The potentially protective role or a role for diminishing protective efficacy for IgA likely depends on 1) antibody specificity (i.e. specificity for epitope on virion particles or infected cells), 2) location (systemic vs mucosal) and 3) form (IgA1, IgA2, secretory IgA, and dimeric IgA). In this proposal, I will define the characteristics of vaccine-induced HIV-1 specific IgA that contribute to the complexity of IgA antibody functions. My overall hypothesis is that the plasma IgA immune correlate of HIV-1 risk identified in RV144, blocks ADCC activity, and although found in the plasma, the IgA correlate of risk is not present in the mucosa due to the different antibody specificities and forms of IgA (dimeric and secretory). In Aim 1, I will isolate and characterize vaccine-elicited IgA from RV144 vaccinees with specificities that correlate with decreased HIV-1 vaccine efficacy. Once isolated from antibody secreting memory B cell cultures, the IgA will be evaluated for their capacity to block IgG mediated antiviral functions. In Aim 2, I will evaluate the differences in specificity and function between vaccine-induced mucosal and plasma antibodies. Since mucosal samples were not collected in RV144, studies have only addressed the ability of plasma IgA to block IgG effector function, and have not addressed whether there are vaccine-induced mucosal IgA and IgG specificities with the potential to interact. Follow-up studies of the RV144 trial, RV305 and HVTN097, used the same prime-boost regimen as RV144 and both mucosal and plasma samples are available for this proposal. I will determine if the plasma antibody specificities associated with the correlates of risk identified in RV144 are present in the mucosal fluid. Finally, I will assess the functional capacity of antibodies found in the mucosal compartments to determine the extent to which they differ from those found in the plasma. The results from this study will provide key information on potentially protective and protection- diminishing mucosal and plasma HIV-specific IgA responses induced by vaccination. Furthermore, the outcomes from this proposal will have a high impact on the criteria utilized for evaluation and selection of optimal vaccine regimens for induction of protective plasma and mucosal antibodies.